Various methods for producing an aromatic polyimide fiber have been heretofore proposed. Particularly, it is well known that the aromatic polyimide filaments can be prepared by a method such that an aromatic polyamic acid resin, which is a precursory polymer of the corresponding polyimide resin, is dissolved in an organic polar solvent, to prepare a dope solution; the dope solution is subjected to a spinning procedure; the polyamic acid in the resultant filaments is converted into the corresponding polyimide, and; then, the resultant polyimide filaments are subjected to a drawing procedure, as disclosed in Japanese Patent Publication Nos. 55-16925 and 42-2936.
However, this conventional method requires the conversion of the polyamic acid into the corresponding polyimide in the course of the filament production. The conversion procedure results in the production of water. Therefore, it is necessary to control the conversion procedure carefully. For this reason, it is difficult to stably produce the polyimide filaments with high reliability.
Moreover, Japanese Patent Application Laid-open No. 50-64522 discloses a special method for producing an aromatic polyimide filament which method comprises dissolving a benzophenone tetracarboxylic acid type aromatic copolyimide in an organic bipolar solvent to prepare a spinning dope solution; extruding the dope solution through a spinneret to form filamentary streams of the dope solution; introducing the filamentary streams into a specific coagulating liquid, and, subjecting the coagulated filaments to a drawing procedure at an elevated temperature.
However, the copolyimide filaments produced by the above-mentioned method are unsatisfactory in mechanical strength thereof.
The inventors of the present invention conducted extensive studies to develop a process for producing aromatic polyimide filaments having a high mechanical strength which is free from the above mentioned disadvantages of the conventional methods. As a result, the invention of the present invention found that when a biphenyl tetracarboxylic acid type aromatic polyimide is dissolved in a solvent consisting mainly of a phenolic compound to prepare a spinning dope solution and the dope solution is subjected to a spinning procedure, the polyimide filaments having an enhanced mechanical strength can be produced. Thus, the inventors of the present invention accomplished this invention.